Fungi adversely affect the health and well being of mankind in numerous ways. The most direct of these ways include a variety of disease processes affecting humans, such as opportunistic infections in immunologically compromised patients, such as patients afflicted with AIDS. The most common treatment currently available for these fungal infections is administration of the antibiotic amphotericin B. However, this compound is highly toxic and doses must be kept to a minimum, especially in critically ill patients. Doses are thus often in amounts insufficient to cure the disease or even halt the infection.
Other harmful effects of fungi include the adverse economic and social effects of plant disease. Purified natural herbicides have at least two advantages; 1) as weed control agents they have longer shelf life, wider range of storage conditions, a broader environmental window for application, less storage space, and a greater ease of application than with living organisms and 2) new microbial phytotoxins may be useful in expanding the number of sites of action of herbicides, in that there is little overlap between the known sites of action of commercial herbicides and of microbially-produced phytotoxins.
Pseudomonas syringae represents a wide range of plant-associated bacteria, some of which are pathogens, while others are weak pathogens or saprophytes. Many biotypes of P. syringae produce one or more bioactive substances that may allow the bacterium to survive in its niche; for instance, on a leaf surface where it must compete with fungi and other bacteria.
A number of novel strategies for the biological control of fungal plant diseases have been developed based on this observation. For example, a transposon-generated regulatory mutant of the wild-type strain of P. syringae 174 (MSU 16H) has been isolated that in culture produces increased zones of fungal inhibition when compared to the wild-type bacterium.
Pseudomonas syringae MSU 16H is a transposon-generated mutant of a wild-type strain that has attracted interest for its ability to make in culture elevated amounts of antifungal metabolites and to confer a greater protection than the wild-type strain in elms infected with Ceratocystic ulmi, the causal agent of Dutch elm disease (Lam et al., (1987) Proc. Natl. Sci. USA 84, 6447-6451). It was tested in elms and shown to confer a greater protective effect than the wild-type strain. More extensive tests on field-grown elms have confirmed the phenomenon of biocontrol at the prophylactic level.
Recently those metabolites, called pseudomycins, have been isolated and their structures have been partially characterized (Harrison et al., (1991), J. Gen. Microbiol. 137, 2857-2865).
Identification of the chemical nature of such toxins, however, has been very limited. Prior to 1970 no correct toxin structure had been published. More recently, attempts have been made to isolate and characterize the molecules responsible for the antimycotic activity of P. syringae MSU 16 H. However, none have been successful because the bioactivity has not been preserved.
The present inventors isolate the complete structure of pseudomycins A, B, C and C'. They are new lipodepsinonapedtides related to syringomycins (Segre et al. (1989) FEBS Lett. 255, 27-31; and Fukuchi et al. (1990) Tetrahedron Lett. 31:1589-1592), syringotoxin (Ballio et al. (1990) FEBS Lett. 269; 377-380; and Fukuchi et al. (1990) Agric. Biol. Chem. 54; 3377-3379) and syringostatins (Isogai et al. (1990) Tetrahedron Lett. 31, 695-698), a group of antimicrobial compounds produced by different isolates of P. syringae pv. syringae.
The inventors show that pseudomycins D and D' are identical to syringopeptin SP.sub.25 -A and SP.sub.25 -D respectively, lipodepeipeptides with 25 amino acid residues produced by some isolates of P. syringae pv. syringae (Ballio et al. (1991) FEBS Lett. 291: 109-112).